To cancel your Nomorobo subscription on an iPhone:
1. Go to Settings > Tunes & App Store.
2. Tap your Apple ID at the top of the screen.
3. Tap View Apple ID. You might need to sign in or use Touch ID.
4. Under Subscriptions, Tap Manage.
5. Tap the subscription that you want to manage. In this case, Nomorobo.
6. Tap Cancel Subscription.
-For more information please check out Apple's Support Page by clicking HERE